In the handling of paper, paper board, or cardboard, stacking apparatus of the aforedescribed type can be provided in which sheets, e.g. individual sheets, folded sheets, groups or collections of sheets, signatures or the like, for example formed by a transverse cutting or slicing machine, are continuously deposited upon a pallet so as to form a stack thereon. The pallet and the stack can be wrapped and, in general, the pallet is used for the convenient transport and storage of the stack of sheets.
To maintain the height of the drop of the sheets during the deposition thereof on the stack more or less constant, the pallet is provided On a raisable and lowerable pallet-receiving platform which can be raised to a level below the location at which a conveyor or feeder delivers the successive sheets, and then continuously or gradually lowered as the height of the stack increases.
German Patent 37 39 194 discloses a stacking apparatus of this type in which, during the replacement of the pallet on the pallet-receiving platform, e.g. when a stacking is complete and the loaded pallet is to be exchanged for an empty pallet, an auxiliary stacking platform is substituted temporarily for the pallet-receiving platform and collects the sheets to form a temporary stack, thereby preventing the sheets from falling in an uncontrolled manner from the delivery device and eliminating the need for cutting off the continuous feeding of the delivery device and thereby limiting the operating speed of the apparatus.
While the stack is formed on the auxiliary platform, therefore, the pallet-receiving platform can be lowered to allow the completed stack and its pallet to be transported away and a new empty pallet to be placed upon the pallet-receiving platform. The new pallet together with the pallet-receiving platform is then raised to a location just below the auxiliary platform and the temporary stack is transferred to the new pallet on the pallet-receiving platform concurrently with a retraction of the auxiliary platform.
To minimize the drop of the temporary stack upon transfer from the auxiliary platform to the new pallet, it is essential that the latter be positioned with great precision relative to the auxiliary platform prior to the retraction thereof at the smallest possible distance beneath the auxiliary platform. This positioning precision requires that the height of the pallet be taken into consideration, and for this purpose, it is known to provide a photocell system for measuring the height of each new pallet.
This measuring method has not been found to be fully satisfactory because of the nature of the pallets and the fact that the pallet may carry a wrapping or protective paper underlay for the stack or may have portions at different levels, for example because a nail may project above the receiving surface of the pallet. Indeed, in many instances the variation in level of the pallet may not be detrimental, except in the case of the transfer of the temporary stack from the auxiliary platform to the pallet.
For example, the protective paper may project above the receiving surface and, although it is not a problem with respect to the receipt of the temporary stack, will give a false indication of the pallet height on measurement by a photocell and thus result in an excessive drop of the temporary stack upon the transfer. In cases in which a nail or the like of narrow dimension may project above the surface of the pallet, it frequently will not be registered by the photocell. For this and other reasons, photocell-measurement systems have been found to be unsatisfactory in many cases.